So you look at a light bulb and it says “100 Watts.” What the hell does that mean? What the Heck is the Difference Between a Kilowatt and a Kilowatt-hour?

Your power company charges your for “Kilowatt-hours” that you use… What the hell is a Kilowatt-hour?!

For starters, a Kilowatt (kW) is just one thousand watts, just like a megawatt (MW) is one million watts. A “watt” (W) is a measurement of POWER. To be precise it’s using one joule per second. However, since no one has uttered the word “joule” since that physics class they took fifteen years ago, we use watts instead of Joules per second, because they’re printed on light bulbs and people have an idea of what they mean. To put it in perspective, a car engine cruising uses about 25,000 watts, which is about 30 horsepower.

But that’s a car… Mechanical energy. We want to talk about electricity since we’re concerned with solar power. With electricity, POWER is voltage time amperage. Or 1watt = 1volt * 1ampere.

A good way to think about electricity is that it’s a lot like water. Voltage is the pushing, or the pressure, and amperage is the flow. A dammed up lake, although it has a hell of a lot of pressure (voltage), doesn’t flow anywhere (zero amperage) so there’s no power (lots of volts * zero amps = zero watts). On the other hand take a super soaker water gun. Super fast stream (lots of amps) but a tiny little compressed reservoir (not a lot of volts). I wouldn’t go so far as to call that “powerful.” Finally, look at a rushing river. Lots of volts, lots of amps, huge power. So wait, Dave, you got off subject, we’re talking about Watts here…

Oh yah, Watts. Ok, so a Watt is energy burned per second. If you flick on a 100 Watt light bulb it’s eating up 100 Joules of energy every second (interestingly, a standard candle is exactly 1 Watt). So now what’s a watt-hour (wH)? Well, don’t get confused when you see a confusing or seemingly incorrect usage of watt or watt-hour in mass media. The two terms are often interchanged and misused.

“Watts per hour” doesn’t make sense because it’s already a measurement of “joules per second.” Does “Joules per second per hour” make sense? No. A Watt-hour is what your power company uses to charge you, and it’s a way of removing the “per second” from Watts. So now instead of talking about Joules per unit of time, you’re just talking about Joules period…. but we call them Watt-hours because no one knows what the hell a “joule” is.

Think of Watts as the speed you’re running and Watt-hours as how far you’ve actually run. A kilowatt-hour is the amount of energy equivalent to a power of 1 kilowatt running for 1 hour. If you leave a 100 Watt light bulb on for 1 hour, you’ve done gone and used up 100 watt-hours.

But your electric company will bill you by the kilowatt-hour, so you’ll get a bill for .1 kWh, multiplied by your per-kWh rate. That means if you run a 100 watt bulb for an hour a day for 30 days in a month, and you pay $.10/kWh, your bill will be for .1 kWh x 30 days x $.10, or $.30.

Ok, I feel like I’ve made that explanation 100 times longer than it should have been. Hope that helped.

Yeah, but… the sun doesn’t shine for 24 hours. The number of peak-sun equivalent hours for San Diego is about 5.7 per day, on average (round up to 6), and you get 6 kWh per kW of solar per day, per kW of panels installed. After losses from wiring and inversion, you’re at about 5 kWh/kW.

The average home in San Diego needs about 6,000 kWh per year, or about 16.5 per day, so you’re looking at a 3.3-kW system to make 6,000 kWh per year. Incidentally, you’re right about San Diego rip-off rates. SDG&E charges an INSANE price for electricity. Those 6,000 solar kWh can save you $1,800 per year.

With a system cost of $3.50 per watt, you’ll pay a little over $8,000 after the tax credit, and those electric bill savings will have that paid off in year 5. Then it’s 20 more years of free electricity from your roof.

This is why people don’t buy solar systems unless they just close their eyes and take a leap of faith. There’s more terminological confusion here than a Democratic Party convention. So, no, it’s not simple to explain these metrics, and that’s just the way utility companies, solar power companies, and government want it.

I think I get it !!! So if my panels are generating 1000 W when I look at the reading, and they do this for six hours, I’ve generated 6000 W or 6kW. So if I do this for 30 days I’ve created 180 kWh, right?

You’ve got it right, Lisa! Numbers in the real world are never that cut-and-dried, as generation is constantly in flux, but when we look at production, we do it exactly like you did, as an average of kWh over time.

WATT: A unit of power. (voltage x current). kWhr: A computational figure only; it is used by power companies to compute billings. EXAMPLE: A light bulb rated at 60 Watts is on continuously, consuming power that is billed at 10cents / kWhr. Since most billings are in kW hr (k=1000) increments, we must convert 60W into kWh: 60W/1000 = 0.06 kWh. NOTE: kWh is not a Watts/Power figure– it is just a figure to be multiplied by the TIME period (one month) AND the RATE (10c per kWhr) for use in a monthly billing, therefore, the final computation will be: 24hr x 30days = 720 x RATE @ $0.10/ kWhr = $7.20 for a 30 day billing.

Okay, so then the bulb is already rated at how many watts it will use in an hour? Meaning if I’m using a 3 watt LED it will only consume 3 watts of energy over 60 minutes worth of time and therefore it’s watt-hour rating is 3?

I’m desperately trying to understand this so I con’t get out in the back country and run out of power halfway through the evening and am stuck until the sun comes up to kiss my solar cells in the morning.

I THINK (please correct me if I am wrong) but if a solar system had 10 200 watt panels and peak solar sun time averaged 5 hours per day AND the inverter and the panels were able to actually OPERATE at the rated output of 2KW (10x200watt panels) then with 5hours per day of Solar sun the system would produce 10KWh (killowat HOURS) of energy available during that time. ALSO if during that day the power is produced then NO power in EXCESS of the hourly draw of 2KW could be drawn from the system. Is this right please?

I have an aquarium pump with a 70 watt capacity. I have a solar panel which outputs 17 volts dc and can charge a 12 volt car battery nicely. If I get an inverter, connect it to the battery charged by the solar panel will it run my pump 24 hours. I live in the Philippines so sun is no problem.

I think KW and KWH are the same, as the load of 100 KW means it will consumes 100 KW if it runs for an hour… so I don’t agree with the above explanationhttp://www.think-energy.net/KWvsKWH.htm
plz refer this link

Think of kW like horsepower. You could have 100 kW panel, but if it’s in the dark, it is not going to produce even 1kWh over the course of an hour. But, in perfect conditions, that is what it could produce in an hour. There are some more complications than that as in what it will, in practice, produce, but you get the idea. Think of kW as horsepower of the system. But the car may be idling or off.

PLEASE EXPLAIN THIS !
People often quote costs/kW when they are really talking about costs/kWh. Since these sound similar, they must be similar. Unfortunately, they are not.
WHAT IS THE DIFFERENCE THEN ?

Can someone help me figure something out? In trying to understand how a sample solar estimate came up with the number of panels it would need. Ok, so a home wants to eliminate 587 Kwhs per month.
-Solar hours per day is 4.9 with the derate factored in.
-Panel/inverter AC CEC watts is 197.8 with inverter efficiency considered
-Kwh/mo/panel is 29
-# of panels required is 20

Sooo, I’m confused because I thought the goal was to have only enough panels to power your house for the Kwh per one day of what you want to elimate. But it seems the number of panels are for a months worth of usage.

Thanks for explaining this difficult concept Dave. Here’s a good excerpt from an article that John Hynes wrote for Renewable Energy World:

Capacity versus Energy

To learn how to compare technologies within a load factor category, one has to understand a number of terms and concepts, like the difference between capacity — measured in kilowatts (kW) or megawatts (MW) — and energy, measured in kilowatts-hours (kWh) or megawatt-hours (MWh). People often quote costs/kW when they are really talking about costs/kWh. Since these sound similar, they must be similar. Unfortunately, they are not.

Capacity for a power plant (kW or MW) is probably best explained with a highway analogy. A 10-lane highway is able to allow more cars to get from one point to another in a given time period when compared to a three-lane highway. Likewise, a 1,000 MW power plant has the ability to put more energy, or MWh, to the grid in a given time period than a 500 MW power plant. The size of the highway is analogous to the capacity, or MW rating, of the power plant. The number of cars that pass from one point to another on the highway during a given time period is analogous to the energy, or MWh, that the power plant delivers during the same time period.

So the more lanes on the highway, the more cars that can pass from one point to another in one hour. Therefore, the larger the power plant’s capacity, the more energy the plant can deliver to the grid in one hour.

Now that we understand the difference between capacity and energy, we can begin to compare the costs of one power plant to another.

I have a 3500sq foot of living space above ground and a 1500sq foot of living space in the basement finished. I average about 3500kwh a month. Does this sound about right, I have 3 gas furnaces heating the home, 3 air conditioners during the summer cooling the home, and of course all the lights and tv’s throughout the house. In the mid of last summer my bill was Almost $600.00 for the month, is there a solar panel/wind turbine system out there big enough that can run my house using this much electricity and if so, how much would a system like this cost?

How much did you pay for electricity last month? If you feel you’re using about as much electricity as your neighbors, I’d compare power bills. If they are similar, perhaps you could jointly approach the power company with a request for clarification. Otherwise, I’d definitely look into a home energy audit. Even if you aren’t hemorrhaging electricity (which I think you might be), your auditor will probably be able to show you how to save more in power expenses than the audit costs. Either way, you save.

Good luck and please give us an update. I’m sure you’re not the only one with a power bill like this.

Dan –
No, my current energy used since the last meter reading is 3640 kWh – the statement shows my daily averages as: Current Month 117; Last Month 126 and Last year as 133 – granted, I am in Arizona and therefore have two a/c/heatpump units on the home, we use these to “zone” the usage. However, we have solar panels to heat our water so I am still baffled at the amount I am using. I can tell from your reaction this is still as extreme as I was thinking. Do you think it would be worth my while to find an energy auditor to check my home out or do you think SRP (the power co) should do some explaining?

That is a whole bunch of electricity per month. Are you propagating anything in your basement? If not (and you aren’t operating a theme park or live in a gigantic mansion), you might want to check your electric bill– again. Perhaps that was the amount you used in the past year.

Seems like your 120 KW turbine would produce about 210240 KWh a year which equates to about £21,000 worth of electricity (probably about $21,000 dollars to taking into account the differences in energy prices over here).

I have an oxygen generator which is rated 480Watts. 4.8 Amps. So if I run it for an hour am I using .48kW ? Trying to figure out how much electricity it uses in a 24 hour period. All help gratefully accepted. Thank you

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